Abstract

A61

Abstract: The success of cancer treatments is limited by resistance to chemotherapies. Furthermore, molecular drug resistance is complex. Consequently, the examination of genetic factors that influence drug metabolism, toxicity, and efficacy have a valuable potential to improve treatment. In addition, cancer cells are characterized by a generalized disruption of the DNA methylation pattern, involving an overall decrease in the level of 5-methylcytosine together with regional hypermethylation of particular CpG islands. We show here a comprehensive genetic analysis to evaluate the predictive value of a set of polymorphisms for 5FU-based chemotherapy clinical outcome. The purpose is to develop an algorithm for treatment optimisation to be implemented at the clinical setting. Methods: Genotyping was performed for known polymorphic variants of genes involved in drug metabolic pathways (TYMS, DPD, MTHFR), drug inactivation (GSTP1) and DNA repair (XPD, XRCC1), by PCR-RFLP method. DNA methylation profiles in tumors were analyzed by using MSP. We prospectively assessed the aforementioned polymorphisms in blood samples from 50 patients undergoing 5-FU/oxaliplatin based treatments, and 50 healthy controls. Clinical outcome was evaluated individually from the beginning of the study. Results: It was found a strong association between the allele with three repeats and variant g (3g) at the promoter of TYMS and a worse response to chemotherapy (95%CI: 2.39-240.63. OR: 24.0; p=0.007) showing more severe side effects, as well as with 6bp insertion allele at the 3'UTR of the same gene (95%CI: 1.17-108.41. OR: 11.25; p=0.03). Moreover, both polymorphisms seem to be at linkage disequilibrium (p-value=0.01), and the haplotype 3R/ins 6bp was significantly associated with a high risk of poorer response to 5FU. However, whether it exists a synergistic effect in relation to response of both polymorphisms remains to be determined. The data suggest that the polymorphisms in the promoter and the 3'UTR insertion of the TYMS gene may be associated with resistance to oxaliplatin/5-FU chemotherapy in the adjuvant setting, as previously described. At the short term, there is no statistically significant association between the remaining polymorphisms and response or clinical outcome, whereas there seem to be a predisposition. On the other hand, we also found strong evidences pointing towards a relation between DPD promoter hypermethylation and chemotherapy response in these patients, as well as a significant association with a decreased global methylation level compared with normal counterpart tissue and with responding patients. We are currently studying a putative positive association between carriers of genotypes containing the MTHFR 677T allele, who show constitutive low levels of 5-methylcytosine in their genomes as we previously described, and tumors in these patients. Conclusions: this study demonstrated that the comprehensive genotyping of TYMS might predict clinical outcome to 5FU/oxaliplatin at the short term. Nevertheless, these markers should be incorporated into long-term prospective clinical trials to confirm its clinical value to individualize treatment for patients with cancer.